How do yellow and red cards work?

Under Law 9, a player who commits foul play must be cautioned, shown a yellow card or sent off.

• Yellow card: the player is cautioned and suspended for 10 minutes. If that player later commits another yellow-card offence, the player must be sent off. Play restarts with a penalty.
• Red card: the player takes no further part in the match and may not be replaced.
• The 20-minute replacement for some red cards is for elite rugby only. Law 9.30 applies it only to elite rugby, where a player sent off for other than intentional and highly dangerous actions may be replaced after 20 minutes of playing time.
• Repeated team offences: when different players of the same team repeatedly commit the same offence, you give the team a general caution, and if they repeat the offence you show a yellow card to the guilty player(s).
• You blow the whistle to caution or send off an offender, and a second time when the penalty or penalty try is awarded.